A novel pathway for hormonally active calcitriol

Calcitriot [1 alpha ,25(OH)(2)D-3] the hormonally active form of vitamin D-3 (D-3) is produced in both renal and extrarenal tissues. Our findings demonstrate that physiological; doses of UVB radiation at 300 nm induce the conversion of 7-dehydrocholesterol (7-DHC) via preD(3) and D-3 into calcitriol in the pmol range in epidermal keratinocytes. The hydroxylation of photosynthesized D-3 to calcitriol is strongly suppressed by ketoconazole, a known inhibitor of cytochrome P450 mixed function oxidases. The UVB-induced formation of calcitriol: in human skin is demonstrable in vivo by the microdialysis technique. These results suggest that human skin is an autonomous source of hormonally active calcitriol. Copyright (C) 2001 S. Karger AG, Basel.